A common type of rock bit for drilling earth formations for forming oil and gas wells and the like has a rock bit body which rotates on the bottom of a drill string. The rock bit body has a plurality of rolling cone cutters mounted thereon for rotation upon rotation of the bit body. Commonly three such cutters are mounted on the rock bit body and occupy a separate 120.degree. sector above the bottom of the hole being drilled. Each cutter has a generally conical configuration and is commonly known as a cone.
For drilling hard formations each cone is equipped with a number of generally circular rows of tungsten carbide inserts which bear on the rock being drilled to apply a high compressive load to the rock and cause its fracture. The cutting action of the rolling cone cutters is typically by a combination of crushing and chipping action of the inserts on the rock formation being drilled. The tungsten carbide inserts must therefore withstand high compressive loads and substantial transverse loading.
The tungsten carbide inserts are commonly mounted in cylindrical recesses in the outer surface of the cone. The inserts are made a few thousandths of an inch larger than the recess and are pressed in to have a tight interference fit to prevent withdrawal during usage.
There are several distinct shapes of tungsten carbide inserts which are standard in the rock bit industry, such as a conical, a double cone, a semi-projectile, and a chisel crest. Such inserts are characterized in having a generally cylindrical inner or base portion which fits into the recess in the rolling cone cutter. The cylindrical base can be slightly tapered. The outer end of such an insert converges to a work surface. The work surface is exposed beyond the surface of the cutter cone and applies the compressive loads to the rock being drilled.
In recent years, rather large size rock bits have been developed for drilling large size wells.
In such applications it is advantageous to use inserts having a diameter of about 3/4 inch or larger. For example, by using such large diameter tungsten carbide inserts fewer rows of inserts and fewer inserts per row are required on each cone. Further the extension of the insert from the surface of the cone can be longer while maintaining adequate strength to avoid transverse failure during the drilling operation. This results in concentration of the drilling load on fewer inserts and consequently greater penetration of the rock being drilled without engagement of the surface of the steel rolling cone cutter on the bottom of the hole. This can result in appreciably improved drilling rates.
Large inserts which extend a greater distance from the surface of the rolling cone cutter can accommodate appreciable wear before it is necessary to withdraw the rock bit from a hole and replace it with a new bit. Thus, by using larger diameter inserts, it is sometimes possible to increase the lifetime of the bit as well as increasing the penetration rate.
An objection to the use of such large diameter inserts is the high cost of material required to form the inserts. The quantity of material required to form an insert increases with the square of the diameter of the insert. For tungsten carbide inserts having a diameter of about 3/4 inch or larger, the incremental expense for forming the inserts is not justified by cost savings realized from using fewer inserts.
It is therefore desirable to provide inserts having a diameter in excess of about 3/4 inch which can be prepared at a cost approaching the cost of smaller diameter inserts. Such economies in manufacture can make large diameter rock bits economically feasible since the drilling rates are significantly improved.
There is, therefore, provided in practice of this invention a large tungsten carbide insert having a large cavity in the cylindrical base portion. Such a cavity extends into the base a distance of at least about 40% of the diameter of the cylindrical base and has a volume in the range of from about 15 to 30% of the volume of the base.
Small dimples in the base of tungsten carbide inserts for earth boring rock bits have been used to provide clearance between the base of the insert and the bottom of the recess in the cone. Such clearance dimples are shown in U.S. Pat. Nos. 3,388,757 to Fittinger and 3,599,737 to Fischer. The depth of such dimples was generally less than about 10% of the diameter of the cylindrical base portion of such inserts and no case is known where the distance the dimple extended into the base exceeded about 25% of the diameter of the cylindrical base. The dimple in such an insert did not extend any appreciable distance into the grip portion of the insert.
No substantial saving in tungsten carbide is provided by such minor dimples in the base of the tungsten carbide inserts. Further, such dimples have been provided in the bases of relatively small diameter tungsten carbide inserts. Dimples of appreciable volume are not feasible in such small inserts.